material of higher refracve index.) Together, these factors reduce the transmission distance of
mulmode ber compared to that of single-mode ber.
Single-mode ber is so small in diameter that rays of light reect internally through one layer only.
Interfaces with single-mode opcs use lasers as light sources. Lasers generate a single wavelength of
light, which travels in a straight line through the single-mode ber. Compared to mulmode ber, single-
mode ber has a higher bandwidth and can carry signals for longer distances. Single-mode ber is
consequently more expensive than mulmode ber.
Exceeding the maximum transmission distances can result in signicant signal loss, which causes
unreliable transmission.
Aenuaon and Dispersion in Fiber-Opc Cable
An opcal data link funcons correctly provided that modulated light reaching the receiver has enough
power to be demodulated correctly.
Aenuaon
is the reducon in strength of the light signal during
transmission. Passive media components such as cables, cable splices, and connectors cause
aenuaon. Although aenuaon is signicantly lower for opcal ber than for other media, it sll
occurs in both mulmode and single-mode transmissions. An ecient opcal data link must transmit
enough light to overcome aenuaon.
Dispersion
is the spreading of the signal over me. The following two types of dispersion can aect
signal transmission through an opcal data link:
• Chromac dispersion, which is the spreading of the signal over me caused by the dierent speeds
of light rays
• Modal dispersion, which is the spreading of the signal over me caused by the dierent propagaon
modes in the ber
For mulmode transmission, modal dispersion usually limits the maximum bit rate and link length.
Chromac dispersion or aenuaon is not a factor.
For single-mode transmission, modal dispersion is not a factor. However, at higher bit rates and over
longer distances, chromac dispersion limits the maximum link length.
An ecient opcal data link must have enough light to exceed the minimum power that the receiver
requires to operate within its specicaons. In addion, the total dispersion must be within the limits
specied for the type of link in Telcordia Technologies document GR-253-CORE (Secon 4.3) and
Internaonal Telecommunicaons Union (ITU) document G.957.
When chromac dispersion is at the maximum allowed, you can consider its eect as a power penalty in
the power budget. The opcal power budget must allow for the sum of component aenuaon, power
penales (including those from dispersion), and a safety margin for unexpected power loss.
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